I got the following question from Dawn F, a fellow worm composter from New York City. She writes:
“Can you spend some time on how to harvest worm castings? I bought a small trommel screen and I gotta tell ya, it took half a day. Even the Worm Inn requires sifting.”
Man, this is kind of a tough one because there's always a trade off.
If you want to harvest lively worm castings without breaking your back, then you either need to be handy, pay someone to be handy on your behalf, or purchase potentially expensive equipment to do your harvesting more efficiently.
I stress the word “lively” because if you're shooting for the moon and want worm castings with a granularity and consistency somewhere between coffee grounds and cocaine, then the equation gets even more difficult and you risk ending up with an end product that will have a much smaller population of beneficial microorganisms.
Factors Affecting Your Worm Castings Harvest
Your ease and effectiveness when it comes to harvesting castings is not just affected by your harvesting equipment, but how you are doing your vermicomposting in the first place.
Factor 1: How You Do Your Vermicomposting
I won't cover all methods here, but suffice it to say, not all vermicomposting methods are equal when it comes to creating “screen-friendly” vermicompost ready to be sifted or screened.
Continuous Flow Through
Price: Varies, but most are somewhat expensive
Description: The continuous flow-through digester (CFT) is an apparatus with rigid sides and a mesh bottom. The worms are fed through the top of the CFT in thin layers. During harvesting, a breaker bar or blade, is dragged across or near the top side of the mesh to cut the bottom-most layer of the vermicompost, which then falls through the mesh to the floor below.
There are many kinds of CFTs to include the excellent product from Michigan SoilWorks as shown in the image at right.
Pros: The most efficient vermicomposting equipment available and castings tend to be “screen friendly.”
Cons: Very pricy to purchase and potentially expensive and difficult to build, even with excellent plans.
Description: The Worm Wigwam is an iteration of a CFT that can be feasible for the home vermicomposter who can stomach the price.
Pros: Probably the largest-capacity, most sophisticated home CFT on the market. Castings should be screen friendly.
Cons: Pricy for a home unit, some users complain of flimsy construction.
Description: A breathable nylon bag I manufacture suspended by an iron frame. It uses the CFT concept as you feed through the zipper top and collect castings via the flat-bottom zipper at the bottom.
Pros: Breathable, durable, simple, and at around $109, a decent trade off between cost and value. Castings are also screen friendly.
Cons: Can be awkward to harvest castings from the zipper bottom.
Description: A series of stacked plastic trays with perforated bottoms to (theoretically) allow upward movement of the worms from one tray to the next, the idea being that once worms have worked through one tray, you begin feeding the next highest tray in order to attract the worms higher.
Pros: Simple design, using an upward migration concept.
Cons: Moisture control can be difficult, somewhat notorious for producing wet, clumpy vermicompost.
Rubbermaid Bin or Bucket
Pros: Inexpensive, easily sourced, and portable.
Cons: Likely to produce a compacted, anaerobic vermicompost that will not be screen-friendly without aerating the bottom and sides to allow air to enter and excess moisture to escape.
Factor 2: How You Screen Your Worm Castings
Again, the following list is not intended to be comprehensive, but rather a range of possible options, from the commercially-produced to the do-it-yourself harvesting solutions.
Like most problems, this too can be solved by simply adding money! Lots and lots of money!
Most large vermicomposting operations use some form of a motorized trommel harvester, basically a cylinder made of one or several screens rotating around a spindle. Normally attached to a 1/2 to 3/4 HP electric motor, trommel harvesters like the Jet Trommel can cost $2000-$5000 dollars or more, depending on the size and complexity.
Another option is the Brockwood Worm Sh*fter manufactured by Worm Farming Alliance member Harry Hopkins. The Sh*fter (no, that’s not a typo) is a very attractive machine that shakes a horizontal screen rather than rotating a cylindrical one. It is small, portable, and has a lot of fans, to include WFA members Gavin Newton of Worm Cycle and Mary Ann Smith of Valley View Worms. Harry advertises it as a 3-way machine capable of handling worms, cocoons, and vermicompost and says it can process 240 pounds of worms and 1000 pounds of castings under the right conditions.
This is awesome! But you know there's a downside, right?
That price tag. But it can be worth it if you've got a lot of castings to screen…..and/or customers to sell them to in order to justify the price.
Price: $79 (with free shipping to the lower 48)
A new option in 2020 is a handmade screener that fits nicely in a mortar tray, which can be bought at Lowe's or Home Depot for less than 6 bucks. Simply dump a gallon or less of vermicompost into the tray and slide it back and forth inside the frame. (This process is made easier by casters on the tray!)
You can have your choice of a 1/4-in or a 1/8-in screen. Choose the 1/4-in if it's the castings you're after and you're OK losing a few worms and cocoons in the process.
If you've got to save every last worm and cocoon, then go for the 1/8th, but expect a degraded volume that will be screened. In all cases, try to dry out your vermicompost before screening for best results!
Flat Screen Tray
And there are many simple ways to harvest castings including a flat screen tray, requiring you to suspend the screen over a large trash can or a some sort of “catch” to receive the castings you're manually jarring loose.
Now as you can imagine, holding a heavy screen over something else while shaking it would be a vigorous core muscle workout at best and at worst, is a good way to earn yourself a trip to the chiropractor for a back adjustment. This is probably one of the reasons why vermicomposting as a business without at least some equipment to help you is going to be more physically difficult than you might anticipate.
One hack to overcome the physical difficulty is to place the flat screen over rollers and maybe even attach handles to the screen to make it more ergonomically sound as you shake it back and forth.
Price: Free, unless you pay for sunlight
I'm not sure whether to classify the light method as a way to extract castings from worms or worm from castings, but this is an ingenious, if tedious, method to separate one from the other.
Worms are repelled by light, and will burrow as best as they can to avoid it. So the idea is to create one or several piles of vermicompost under a bright light and wait for the worms to dive deeper into the pile to avoid the light, at which point you scrape the top and sides of the vermicompost they leave behind. Rinse and repeat this process until you're left a wiggling pile of worm meat and a bountiful harvest of castings/vermicompost.
An extension of this concept is to combine the light method and the use of a flat screen tray as mentioned above. I can't say I can vouch for this personally, but the concept is straightforward….
- Fill your tray with worms and vermicompost.
- Place the tray over a mortar bin, Rubbermaid bin, or anything you would like your worms to end up in.
- Place a bright light over the tray.
- Keep scraping the top layer of the vermicompost as appropriate
- Eventually, the worms, repelled by the light and with nowhere else to go, will drop themselves into the bin below.
An excellent video can be found below. Matthew covers many of the techniques we've talked about to include trommel harvesting, flat screens, and the light method.
The Onion Bag Technique
Now this is definitely a way to extract worms rather than castings from a worm bed. I first learned about this from Bentley Christie, who wrote about it years ago (though I'm having trouble finding the link). Basically, you throw some fairly rich worm foods in an onion bag or any other bag that has a very loose mesh and you place it in the your worm bin and lightly cover with vermicompost. (This will probably work better if you place the onion bag in a slightly underfed bin.)
Come back 2 days later and retrieve the bag and maybe even some of the vermicompost just adjacent to the bag. Both should have a dense population of worms that you can use to start a new worm bin.
The vermicompost you are left with will definitely have some worms and cocoons left behind as well as some undigested matter, which probably STILL makes you wonder how you're going to screen that mess.
Well I might have welcome news for you below.
Maybe You Don't Need to Screen Castings At All!
This is where I potentially make a moot point out of the previous 1300 words, but it may be really good news for folks like Dawn, who would like to get out of bed without a super dose of Advil!
While having a nice, uniform cast may seem like a great goal, one question you should ask yourself is whether or not you really need to be screening your vermicompost in the first place. If you need a uniform product to meet a customer’s needs or can't tolerate any unprocessed material in your final product, then obviously the answer is “yes.”
You can get this at a cost by using a 1/8-in screen to get really finely screened castings and prevent cocoons from escaping. A 1/4-in screen will work more quickly but allow more cocoons and a slightly more heterogenous mixture through.
But you risk stressing and/or destroying the very microbes that inject life back into the soil. This is especially true of fungi, which can grow in filaments called hyphae and are likely to be torn apart by the screening process.
You can also speed up your harvesting by drying your castings considerably before you run them through your screener or harvester. But again, the drying process also destroys the very microorganisms who rely on moist conditions to multiply, let alone survive.
But if you're maintaining an environment where worms are multiplying rapidly and you don't need to worry about a few worms and cocoons in your finished product, then you might do the following:
- bait the top of your worm bin surface with worm chow, pumpkin, melon, or other foods the worms find attractive
- or bait one end of your bin if you have an elongated worm bed where moves can migrate a few feet laterally
- wait 24-48 hours and scrape (what is hopefully) a worm-rich layer of vermicompost from the top of your bin
- and use the remaining vermicompost as-is
I asked two trusted Worm Farming Alliance soil experts, Nina Folch-Torres of Microbes in My Soil, LLC and Heather Rinaldi of the Texas Worm Ranch what they thought of using unscreened vermicompost and both gave it the thumbs up!
Nina responded that she doesn't screen her vermicompost before applying it to her garden, mentioning that her own observation of unscreened vermicompost indicates that “it's a much surer way of protecting that biology.”
Heather added it's also a great way to add much-needed soil organic matter (SOM) to the soil, cautioning that visible sources of nitrogen like food waste or manure should not be present.
So once you are able to identify when your vermicompost is finished and ready for harvest, I think you have the green light to apply your vermicompost to your soil without ever going through the backbreaking or “bank-breaking” process of finely screening it.
Your Choices: Spend Money, Spend Labor, or Accept a Useful, but Less Pretty End Product
I get it.
A nice, dark, uniform pile of black gold can make your heart sing. It's like the satisfaction of hand-squeezing orange juice, or finishing that handmade quilt you labored over for months.
But you only have to squeeze so many oranges to make an adult-sized glass of OJ. And unless you're like Adam Sandler's Grandma in Happy Gilmore and have to meet a quilt-production quota, your quilting hobby is just a labor of love.
But if you're faced with 10 square feet of finished worm compost, and the castings separation is tedious, expensive, or backbreaking (or maybe a combination of the 3), then I would highly consider not screening at all.
I know the potential cost of not removing every single worm and cocoon. If you're growing worms to make money, I've written how costly that can be in theory.
But if you're a home gamer like Dawn, I would just remove what's easy to remove and be happy with an 80% solution.
I like to say that vermicomposting is like sex. Even if you're not doing everything correctly, it's still really good.
It's kind of a joke, but it's also true. And if the difficulty or expense of producing uniform castings is going to prevent you from vermicomposting in the future, then I think you're better off just using unscreened vermicompost. The product is less aesthetically pleasing than a uniform pile of dark brown soil cocaine, but you retain all – or more – of the potentially immense benefits to your soil and plants.
The 2020 Vermicomposter's Starter Guide
Your roadmap to a successful beginning in vermicomposting.